In a conventional optical communication system, for example, information transmission using a modulation system that utilizes light intensity, such as an OOK (On-Off Keying) system, is performed.
On the other hand, along with the recent realization of a high speed optical communication system and reduction in its cost, a differential phase shift keying (DPSK) system using optical phase information is attracting attention, because of being excellent in non-linear durability and being expected as having an effect of improving the sensitivity to about twice (3 dB) as compared with the OOK system.
For decoding an optical signal transmitted in accordance with the DPSK system, an optical decoder using, for example, a 1-bit delay unit (hereinafter, referred to as “MZ-type 1-bit delay unit”) of the Mach-Zehnder (MZ) type is used. The MZ-type 1-bit delay unit decodes an optical signal transmitted using the electrooptical effect of a dielectric waveguide formed of lithium niobate (LiNbO3) or the like.
In the MZ-type 1-bit delay unit, in a case where a phase difference between continuous bits is 0, an optical signal is output to one port called a constructive port, and in a case where the phase difference is π, an optical signal is output to the other port called a destructive port. Optical outputs from both the ports are received differentially at an optical receiver called a balanced receiver. Thus, in the optical reception device having an MZ-type 1-bit delay unit, differential reception using the balanced receiver is used, whereby the reception sensitivity is enhanced.
However, in a case where the phase of the MZ-type 1-bit delay unit or the wavelength of an input optical signal vary due to a change in environment such as temperature, the reception sensitivity is degraded. Thus, there is a conventional problem of stabilizing the reception sensitivity even in the case where the environment such as temperature varies.
As a prior art for solving the above-mentioned problem, a process for stabilizing the phase of the MZ-type 1-bit delay unit is proposed, which performs control so that a bit rate frequency component of an optical signal received differentially at the balanced receiver becomes maximum (e.g., see Non-Patent Document 1).
The light reception device shown in Non-Patent Document 1 is composed of a DPSK receiver including a 1-bit delay unit, a balanced receiver, and the like, and a control system including an RF power branching unit, an RF power detector, a phase control circuit, and the like. A part of a high-speed electric signal immediately after the balanced receiver is branched at the RF power branching unit so that the RF power thereof is measured by the RF power detector. As a result, in the optical reception device, a phase control function of the Mach-Zehnder type 1-bit delay unit is controlled so that the measured RF power becomes maximum, whereby the reception sensitivity is stabilized.
Further, as one of the other processes, a process for stabilizing the phase of the MZ-type 1-bit delay unit that controls a phase based on a DC current flowing through the balanced receiver is also proposed (e.g., see Non-Patent Document 2).
A reception device of an optical transmission system shown in Non-Patent Document 2 is composed of a DPSK receiver including a 1-bit delay unit, a balanced receiver, and the like, and a control system including a DC current detector, a phase control circuit, and the like. A DC current component of the balanced receiver is detected by a DC current detector, and a phase adjusting function provided in the Mach-Zehnder type 1-bit delay unit is controlled based on the detected DC current value, whereby the reception sensitivity is stabilized.
Non-Patent Document 1: Biljana Milivojevic et al, “Practical 40 Gbit/s CSRZ-DPSK transmission system with signed online chromatic dispersion detection” ECOC2003, TU364
Non Patent Document 2: Eric A. Swanson et al, “High Sensitivity Optically Preamplified Direct DitecOtion DPSK Receiver with Active Delay-Line Stabilization” Photonics technology letters, vol. 6, 1994